The Role of Vitamin B12 in Methionine Biosynthesis in Avian Liver

Effect of methionine on the metabolic fate of liver folates in vitamin B12-deficient rats

1. Hepatocytes isolated from vitamin B12-deficient and vitamin B12-supplemented rats were maintained in primary culture and were used to study the effect of methionine on the metabolism of [3H]folic acid and [5-14C]methyltetrahydrofolic acid.

2. Vitamin B12levels were reduced by approximately 75% in the hepatocytes from the deficient animals. Total folate and methyltetrahydrofolic acid concentrations were also significantly reduced.

3. There was no significanct difference in the uptake and retention of added [3H[folic acid and [5-14C]-methyltetrahydrofolic acid between the hepatocytes of the two groups. The incorporation of14C into phospholipids was reduced by approximately 60% in the vitamin B12-deficient hepatocytes (P< 0.001).

4. The addition of methionine to the culture medium doubled the uptake and retention of3H in both groups, but it did not change the amount of water-soluble14C Compounds. In the vitamin B12-deficient hepatocytes mainly methylated folate increased, whereas non-methylated folate increased in the hepatocytes of the control animals. A tenfold increase of14C incorporated into phospholipids was found in both groups after methionine was added.

5. Demethylation of methyltetrahydrofolic acid, the intracellular retention of folate and the utilization of liberated methyl groups, for example in the methylation of phospholipids, were highest in the presence of both methionine and vitamin B12suggesting an intimate co-ordination between these two substances in the regulation of folate metabolism.

Effect of vitamin B12 deficiency on phosphatidylethanolamine methylation in rat liver

1. In vitamin B12 deficiency the activity of tetrahydropteroylglutamate methyltransferase (EC 2.1.1.13) is depressed and the synthesis of methionine is reduced. Because the methyl group of methionine is largely utilized for the methylation of phosphatidylethanolamine, we investigated the effects of vitamin B12 deficiency on phosphatidylcholine synthesis. 2. The incorporation of injected [14C]formaldehyde into liver phosphatidylcholine was reduced by approximately 50% in vitamin B12-deficient rats. Also the corresponding incorporation of 5-[14C]methyltetrahydrofolic acid tended to decrease. The findings are consistent with a lower conversion of these precursors to methionine. 3. The effect of the deficient methyl-group supply on phosphatidylcholine synthesis was also investigated by the injection of [14C]ethanolamine. The amount (%) of lipid-14C recovered in phosphatidylcholine was significantly reduced in vitamin B12 deficiency. 4. Chemical analysis of liver phospholipids showed that the vitamin B12-deficient rats had a higher proportion of phosphatidylethanolamine and a lower proportion of phosphatidylcholine, indicating that the impaired synthesis of phosphatidylcholine by methylation leads to changes in membrane phospholipid composition.

Ribonucleotide reductase in blue-green algae: dependence on adenosylcobalamin

Ten species of freshwater blue-green algae exhibit an adenosylcobalamin-dependent ribonucleotide reductase, thuse explaining the requirement for cobalt by these organisms. The evidence suggests a phylogenetic affinity between the cyanophytes and bacteria, such as Clostridium and Rhizobium, and the euglenoid flagellates, which also use the cofactor-dependent reductase. In contrast, the ribonucleotide reductase reaction in the few green algae surveyed shows no dependence on cobalamins.

Variable response of bone marrow to feeding DL-5-formyltetrahydrofolate in pernicious anaemia

It has been suggested that the megaloblastic anaemia in pernicious anaemia is due to inadequate intracellular concentration of monoglutamyl folates other than methyltetrahydrofolate caused by diminished conversion of methyltetrahydrofolate to tetrahydrofolate (methylfolate trap). To test this, we have increased the concentration of methyltetrahydrofolate in the plasma of six patients with pernicious anaemia by feeding DL-5-formyltetrahydrofolate. The effect of therapy on bone marrow morphology and routine haematologic parameters was measured. Of two patients receiving 800 mug/d of DL-5-formyltetrahydrofolate, one had a significant response; of four receiving 6 mg/d, one converted erythroid maturation to normoblastic, and in two others some improvement was noted in levels of neutrophils, platelets or reticulocytes although marrow morphology remained megaloblastic. Response did not correlate with the degree of elevation of plasma folate. In patients receiving this therapy, slight increase of methylcobalamin in plasma may have occurred (P less than 0.05). These observations support ineffective utilization of methyltetrahydrofolate as the major cause of megaloblastic anaemia in pernicious anaemia, but indicate that the degree and location of block varied in different patients, and in different precursor cells of a single patient.

The relationships between vitamin B12 and folic acid and the effect of methionine on folate metabolism

The relationship between vitamin B12 and folate and the effect of methionine on folate metabolism during B12 deficiency in rats is best explained by the prevention of the accumulation of 5-methyl-H4PteGlu by vitamin B12 and/or methionine. Although several points remain to be clarified, the 'methyl trap' hypothesis provides the most satisfactory explanation for the relation between vitamin B12, methionine and folic acid. This concept is extended by the hypothesis that H4PteGlu is the most active substrate for pteroylpolyglutamate synthetase, and thus accounts for the effect of methionine or vitamin B12 increasing liver folate levels.